The present invention relates to cooling systems for auxiliary power units on airplanes and, more particularly, pertains to the passive cooling of the components and oil of such units and the enclosure ventilation of such units.
Large aircraft often use an on-board auxiliary power unit (APU) to provide electrical power and compressed air for systems throughout the airplane. When the aircraft is grounded, the APU provides the main source of power for environmental control systems, hydraulic pumps, electrical systems and main engine starters. During flight, the APU can supply pneumatic and electric power.
Auxiliary power units are generally small gas turbine engines, often mounted in the aft tail section of the aircraft. They require a certain amount of cooling air, and are lubricated by oil that is generally cooled by an oil cooler which also requires cooling air. Active cooling systems are usually employed to provide this cooling air, and are typically comprised of an active fan used to push air through the oil cooler and across auxiliary power unit components. These fans are driven at high speeds by the APU through a complex shaft and gear assembly. The mechanical complexity and high operating speeds of these fans increases the possibility of failure. Active fan cooling systems therefore can significantly reduce the reliability of an auxiliary power unit.
While APU passive cooling systems which eliminate the need for active fan cooling systems are well known, they all generally draw cooling air into the APU compartment, before it is drawn through the air cooled oil cooler. This arrangement causes the cooling air to be heated up in the compartment before it reaches the oil cooler, and therefore, oil cooling is not optimized. U.S. Pat. No. 5,265,408, for example, discloses a method and apparatus for cooling a compartment mounted gas turbine engine comprising a first exhaust eductor within which is mounted an oil cooler, and which incorporates a mixer nozzle to entrain cooling air flow first through the APU compartment and then through the oil cooler. Surge bleed flow from the load compressor is discharged into the exhaust eductor. Ambient air is received into the compartment through a second exterior eductor inlet.
U.S. Pat. No. 5,655,359 similarly discloses an APU passive cooling system wherein cooling air for the oil cooler is drawn from the compartment. An inlet scoop in the engine air intake duct used to divert a portion of the air flow into the APU compartment. This air is used to cool the engine before being drawn through the oil cooler, mounted in a vacuum duct, by a lobed mixer which acts as an aspirator.
U.S. Pat. No. 6,092,360 discloses an APU passive cooling system in which cooling air is drawn into the engine compartment through an opening located in the rear of the aircraft. An eductor mounted before the exhaust duct of the engine, draws compartment air through the oil cooler, which in turn draws atmospheric air in through the aft opening.
Thus, while these patents provide for cooling of an auxiliary power unit without the use of a mechanically driven fan, they all teach systems which draw cooling air for the oil cooler from the APU compartment. A need exists for an auxiliary power unit passive cooling system that can provide enhanced oil cooling capabilities by directing exterior cooling air, through ducts, directly to the oil cooler, and which is nevertheless adaptable enough to be able to provide damage protection from foreign objects and be combined with the engine compressor surge bleed flow to provide improved airflow through the oil cooling heat exchanger.
It is an object of the present invention to provide an improved cooling system for an auxiliary power unit on an airplane.
It is also an object of the present invention to provide a simpler cooling system for auxiliary power unit engine oil and external components which does not require moving parts and does not include a cooling fan.
It is a further object of the present invention to provide improved cooling of the oil in an auxiliary power unit by providing enhanced cooling airflow through the heat exchanger.
Therefore, in accordance with the present invention there is provided a passive cooling system for an auxiliary power unit installation on an aircraft, comprising an auxiliary power unit housed within a nacelle of the aircraft, the auxiliary power unit comprising at least a compressor portion of a gas turbine engine and an oil cooler contained separately within the nacelle, an engine exhaust opening defined in the aft portion of the nacelle and communicating with the gas turbine engine, at least a first cooling air inlet duct communicating with a second opening defined in the nacelle and with the compressor portion, the oil cooler located within a second duct communicating with the exterior of the nacelle and the engine exhaust opening whereby exterior cooling air, and engine exhaust ejected through the engine exhaust opening entrains cooling air through the second duct to the oil cooler, providing engine oil cooling.
In accordance with the present invention, there is also provided a passive cooling system for an auxiliary power unit installation on an aircraft, comprising: an auxiliary power unit housed within a nacelle of the aircraft, the auxiliary power unit comprising at least a compressor portion of a gas turbine engine and an oil cooler contained separately within said nacelle; an engine exhaust opening defined in the aft portion of said nacelle and communicating with said gas turbine engine via an exhaust eductor assembly; said exhaust eductor assembly being in fluid flow communication with a compressor surge bleed duct; at least a first air inlet duct communicating with a second opening defined in said nacelle and with said compressor portion; and said oil cooler located within a second duct communicating with an opening other than the engine exhaust opening of said nacelle and with said engine exhaust opening, whereby exterior cooling air and engine exhaust ejected through said exhaust eductor assembly, entrain cooling air through said second duct to said oil cooler, providing engine oil cooling.
In accordance with a more specific embodiment of the present invention, the engine air inlet includes a first duct portion, and the second duct is bifurcated from the first duct portion and extends downstream from the first duct portion with a third duct portion also formed downstream of the first duct, the third duct portion communicating with the compressor portion and the oil cooler located within the second duct portion providing direct exterior cooling air to the oil cooler.
In one embodiment, contamination of aircraft environmental control system air is prevented by an air inlet splitter, which isolates the load compressor gas path. Protection against damage from foreign objects, for the powerplant, may be provided by a bypass duct located in-line with the first air inlet duct, and a scavenger discharge duct and outlet which expels harmful foreign objects from the aircraft. The nacelle is provided with a rear exhaust opening, and at least a second opening for the outside air intake. The third air inlet duct portion directs the air from the air intake to the engine compressor portion. The auxiliary power unit comprises a gas turbine engine having both load and core compressors and a compressor surge bleed valve and duct. The oil cooler may comprise an air-to-oil heat exchanger. The engine exhaust ejector creates a depressurization in the nacelle or in the exhaust eductor assembly, which results in the entrainment of cooling air through the heat exchanger and through the nacelle. In at least one embodiment, a dedicated small opening in the exhaust eductor assembly permits nacelle ventilation.